Constrution - Moisture control strategies

Moisture Control Strategies

As we've improved energy efficiency of houses, a side effect has been that we've also increased the risk of mold.  In the past, high air leakage rates resulted in very low indoor humidity and hence very low chance of mold. Along with tightening up buildings, builders have also moved to progressively less permeable sheathings, that is from 1x8 boards, to plywood, to OSB.  As the air leakage has gone down, indoor humidity tends to go up, although how much depends heavily on occupant behavior and weather conditions as well as building design.  Because there are so many variables, this section is about reducing risk rather than a guaranteed elimination of any problems. Which techniques make sense will also depend heavily on climate--in particular whether seasonal drying is an option or not.  As a builder, the goal is to provide a reasonable safety margin against mold without driving the cost up too much.

The challenge of moisture control is not only that the conditions vary, but that every building assembly (ie walls, ceiling etc) is a multi-layer sandwich of materials of various permeability and because each layer itself is made of individual pieces, the permeability of each layer likely varies at the joints.  To complicate matters further, the permeability of a layer might also vary due to conditions, in particular its moisture content.  Buildings also change over time:  someone cuts holes in the drywall to install or move a wire, and then it doesn't get repainted with vapor barrier paint, or maybe carpenter ants or other bugs chow various tunnels in your foam insulation, or maybe some spray foam came loose, or a piece of tape, or maybe you're remodeled and someone didn't put something back functionally the way it was. Building don't get built perfectly and even if they did, they're not likely to stay that way

In choosing moisture control strategies, its best to keep in mind the saying "Anywhere water can go, it will."  The questions the builder/designer must  answer are: how strong is the driving force, where is, how long will it operate for, can I keep it from condensing, can I allow it to pass thru wit no harm or dry seasonally?

This section requires a significant amount of background info:

Health and Moisture - relative humidity, absolute humidity, humidity and temperature; moisture movement; the health implications of humidity.

Health and Ventilation - why ventilate: fresh air, dilution air; humidity implication; air quality and filtration; infiltration and ventilation; passive ventilation; backdrafting

Condensing potential (mold)  -  temperature gradients, condensing surfaces

Vapor movement - how vapor moves; permeability; moisture reservoirs.

Ventilation - types of ventilation systems

Air sealing - how to reduce air leakage in buildings

Water - controlling the entry of liquid water

Strategy 1: Reduce the amount of water

Obviously the place to start is making sure there isn't excess water in the house. For the builder, this requires a bit of attention to detail--things that you might have gotten away with a long time ago that now require more attention: getting all the flashing details right, keeping water out the basement,  and  eliminating water moving up thru the foundation/slab by capillary action

It also means that ventilation fans are required near sources of moisture--kitchen, laundry and baths--but of course how much of a factor they are depends on how much moisture the occupants generate and whether the fans actually run (given that the controls for these fans is typically manual)  Of course, these fans might also be part of a whole house ventilation system.  The challenge here is that typical ventilation control systems are not triggered by sensors, but rather run on some time schedule, and that their primary purpose is fresh air, not moisture reduction (and of that, it's primarily dilution air--see full discussion on ventilation requirements), so humidity levels are just a secondary concern, but unfortunately there really isn't a correlation between the rate occupants add pollutants to the air and the rate humidity is added. Luckily the typical ventilation rates are enough to keep indoor humidity from getting too high.

Note also that infiltration matters (in both good and bad ways).  First, infiltration increases the ventilation rate, although that varies with the weather from a significant amount to nearly zero.  The tighter the house, the less frequently the infiltration rate adds significantly to the ventilation rate.  Second, of course, its infiltration that is the main  cause of mold--or rather infiltration thru assemblies where the air slows down long enough and cools enough to deposit moisture.

The challenge of ventilation is both that it is an energy penalty (although using an HRV will eliminate most of the penalty at the cost of higher initial expense), and that ventilation affects humidity levels.  The colder it is outside, the more ventilation will result in lower indoor humidity, and the hotter it is, the reverse will be true (that is assuming air conditioning).  So up to a point, in the winter, ventilation keeps the indoor humidity from  getting too high.  If the indoor humidity is constantly too low, you can either add a humidifier, or use an ERV (which returns most of the moisture back to which ever side is damper), but once you use the ERV, your ventilation system loses most of its ability to dry the air, which means you are now relying on the occupants using spot ventilation fans and/or simply not generating much moisture.  Given that the ventilation rate is generally dictated by dilution air requirements and that this rate is generally high enough to cause significant drying during winter.

Strategy 2: Reduce Air Flow Thru Insulated Cavities

The most common cause of mold growth is due to the movement of moisture thru a permeable material (such as insulation) to a less permeable material (such a wood sheathing) that is cold enough that the moisture condenses.  The biggest source of moisture is air movement, so hence the strategy is to eliminate air movement (see air sealing).   Note that from the mold perspective, we don't care about overall air leakage--any air that flies out of the building faster than it can condense doesn't cause a problem--hence we don't care if the building itself it airtight, only that the assemblies are.  So while leaks around windows and doors, out ventilation ducts or other penetrations will come with an energy penalty, for the most part there is not an additional mold issue.  It also doesn't really matter which side of the assembly the air barrier is, just as long as its continuous--generally pressure differences only exist before inside and out.

The limitation of this strategy is that no air seal is perfect, hence some moisture will leak in, and also its likely that some moisture will migrate in by vapor transport.  For this strategy to work, any moisture that gets in will need a way back out.

Reducing air flow works in both heating and cooling climates.  It also has the advantage that it reduces heat transfer.  Finally, air movement thru insulation turns it into an air filter of sorts, which means any dust, pollen and any other airborne pollutants will end up trapped in the insulation.

Strategy 3: Keep the condensing surface above the dew point

Since it is nearly impossible to completely keep moisture out, another strategy is to just keep it as far form the dew point as possible.  Typically the way this is done is that enough insulation is put on the exterior side of the sheathing (since the sheathing is typically the place where condensation occurs) so that the condensing surface (ie that would typically be the sheathing) stays well above the dew point most of the year.  Note that in a hot humid climate, the wall would have to constructed in reverse from that in a cold climate.

While this strategy is usually implemented by adding a substantial amounts of rigid insulation to the exterior, it could also be done by adding framing (Larsen truss for example) to the exterior and filling the cavity.  Both achieve the same effect--that is the sheathing, which is not especially permeable, stays above the dew point.

With this strategy the wall cavity can still be filled with permeable, fluffy insulation, but there must be enough exterior insulation to keep the sheathing warm enough so that any moisture will never (or rarely be near the condensing point).  Alternatively the fill insulation is not necessary, although leaving it out will result in a somewhat lower overall R-value.

Strategy 4: Use Moisture Reservoir Materials

To prevent mold, we don't need to completely eliminate moisture migration, just limit it enough so that there is no damage.  Thankfully common building materials help in that they can absorb quite a bit of moisture, so condensation is reduced by what they absorb.  Both wood and cellulose insulation are good absorbers and so are straw bales (hint: they're all cellulose), but most other building materials are not.  As long as the total moisture content in those materials stay low (below 15%, or at worse below 20%), there will be no mold.  The downside of this is that wood moves when it absorbs water, particularly larger dimension lumber like 2x10s, which can widen by up to 1/4". Note also that uncoated masonry siding products (brick, stone, stucco) are also moisture reservoirs although they only grow mold on the surface due to nutrients in dust.

The obvious limitation is that materials can only absorb so much moisture, so this strategy only works if the total amount of moisture is fairly small, which means that there isn't a strong vapor drive, and that there is very little air transported moisture. This limitation also implies that this strategy only works if the moisture drive is reversed seasonally--that is what happens yearly is that a small amount of moisture gets in, the materials store it, and then in the reverse season, its dries back out.

Strategy 4B: Avoid Moisture Reservoir Claddings

This strategy really only applies to climates where rain is followed by hot sun. The issue is that if these materials (typically masonry) get very wet and then if the sun shines on them enough to heat them above indoor temperature, this solar heating can create a fairly strong inward vapor pressure drive until they dry out.  If this is repeated frequently, mold damage is likely.

There are multiple ways to mitigate this, the main one being to install these claddings over a well-vented air space (rain screen). The other would be use paint to eliminate the moisture absorption and to use an overhang to keep the sun off the siding.

Strategy 5: Allow Drying

There are really four versions of this strategy:

  • Block all moisture movement (ie no drying at all)
  • Drying to the inside
  • Drying to the outside
  • Drying to both sides

First keep in mind that, regardless of which strategy is chosen here its still critical to keep airflow thru all assemblies to a very small amount since airflow brings a lot of moisture with it.

This strategy is only about vapor transport out of assemblies, not any air driven drying, because unless the pressure differences that create airflow are controlled (eg by mechanical ventilation), there is no reason to expect airflow will go thru the assembly in the direction that creates drying1. Because moving air will typically bring a large amount of moisture with it, no type of drying can compensate for a leaky assembly unless the leaking air is almost always fairly dry.

Even within a given climate, whichever strategy is chosen here will likely be combined with another strategy.

There are two things to keep in mind with this strategy:

  1. If you design to let moisture out, it can also go in.  So its a matter of examining the magnitude of both for your climate, and picking a strategy based on that.  The caveat to this is that there are now "smart membranes" on the market that change permeability--the idea being that they block winter inward moisture drive and then allow more out in the summer.
  2. Whatever you build, you end up with one of the options here--that is either the assembly can dry or it can't, and by not choosing, you're just getting a random choice.
Block all moisture (no drying)

In this strategy, in addition to blocking air flow thru all assemblies, we install vapor barriers on any side that has vapor drive--certainly the inside in heating climates, and the outside in cooling climates, but maybe both. 

The downside to this is that no barrier is perfect, and even if you could construct it perfectly, everything eventually fails. So "block all moisture" really means "block almost all moisture", and leads to the question: will the moisture that gets in come back out before it does any damage?  This depends on both the magnitude and duration of the driving force that puts the moisture in the cavity versus what drives it back out.  It also depends on where the imperfection is: for example if the moisture drive is generally outward (as it would likely be in a cold climate) and the imperfection is on the inside, then likely the wall will never dry, but if its outside, then likely it will always dry. 

Given that there will likely be a small amount of wetting/drying in this strategy, the builder should consider other strategies as well, or use materials that are less susceptible to mold damage.

Dry to the inside

In this strategy, the interior wall surface is at least semi-permeable and the exterior is of low permeability.  This would be the default in cooling dominated climates where the vapor drive will generally be inward. Note that in hot dry climates, the drive is usually not large: at 105°F and 15% RH, cooling the air to 77°F still keeps the RH below 40%.  As long as not too much moisture gets by the outside vapor retarder, it can dry to the cooler interior--although the amount of drying is dependent on the difference in absolute humidity between the interior and inside the cavity.

In mixed climates, as long as the heating season is short enough, there can still be drying to the interior the rest of the year.--that is as long as the vapor drive reverses seasonally.  Whether this keeps the assembly dry enough depends on whether the drive outward matches the inward drive and/or whether the durations of these drives match.  How effective this will be depends on the absolute humidity of the interior being lower than in the assembly cavity--meaning that the relative humidity will also be quite low.  While these conditions are likely true in the moderate winter part of the dry-summer western US, they're not likely true elsewhere.

Most northern area building codes require an interior vapor retarder, and some require a vapor barrier.  While these reduce the amount of moisture going into the wall during heating season, they also reduce drying.  For that reason, in moderate climates, especially with dry summers, this vapor barrier might not be a good idea---it really depends on the relative difference between how much moisture is likely to move in the cavity, versus how much will likely move back out.

Dry to the outside

In this strategy the exterior is permeable and the interior isn't.  This would be the default for heating climates. This is just the reverse strategy to "dry to the inside", which means that in cooling climates you still could get some drying to the outside during the winter.

The same seasonal moisture drive reversal would allow some drying to the outside in cooling climates.

Given that most siding materials are not especially permeable, the best way to improve drying to the outside is to put the siding on a rain screen.

Because warm air and moisture (think showers and cooking, but also just temperature stratification) moves upward, the issue with roofs is more severe to the point that few jurisdictions will allow unvented roofs.  A vented roof is conceptually like a rain screen, only the venting is generally on the insulation side of the sheathing. The venting allows any moisture that sneaks thru to dry to the exterior.   For an unvented roof to prevent condensation, it will need the same treatment as a wall--that is, layers of insulation outside the sheathing so sheathing stays above the dew point, or alternatively the entire assembly needs to be a barrier to air and vapor movement (as with an SIP).  

Dry to both sides

In this strategy, both interior and exterior are vapor permeable (at least somewhat).  The catch here is that the strategy fails if there is a condensing surface inside the wall and too much moisture gets thru (that is the inward drive is faster than the outward drying).  So either eliminate any condensing surface OR keep the condensing surface above the dew point OR be confident that conditions rarely occur where moisture builds up in the condensing surface (and that if it does, the material will dry before mold can grow).

Given that strong, permeable sheathings are not commonly available, in areas with high wind loads and/or earthquakes most houses will be sheathed in plywood or OSB, both of which are only semi permeable, but are able to hold some moisture (OSB has more glue in it, so its less permeable and hold less water).


Whatever set of strategies you choose, it will be dependent on both your climate and the building materials you choose--the issue there is that some materials are very permeable and some aren't.  For example, if you want to use closed cell spray foam, then the "drying to both sides" strategy is not an option, because the foam is not permeable.   Likewise, if your building material is straw bale, which is fairly susceptible to mold damage, then likely the only reasonable strategy will be "drying to both sides".

No air or vapor barrier is ever likely to be perfect, so ideally there  are more significant imperfections on the side where drying is more likely to occur.  Experience with typical wood frame buildings is that in most climates the wood's ability to absorb moisture makes them somewhat resilient, although climates with high summer humidity are certainly more challenging.


1: of course, for years we did, but there was a big energy penalty that went with it and it meant that indoor humidity was very low all winter.